Fuel cell system

ABSTRACT

A fuel cell system includes a fuel cell module for generating electrical energy by electrochemical reactions of a fuel gas and an oxygen-containing gas, and a condenser for condensing water vapor in an exhaust gas discharged from the fuel cell module by heat exchange between the exhaust gas and a coolant to collect the condensed water and supplying the collected condensed water to the fuel cell module. The condenser includes an air cooling condensing mechanism using the oxygen-containing gas as the coolant, and a water cooling condensing mechanism using the hot water stored in the hot water storage unit as the coolant. The air cooling condensing mechanism includes a thermoelectric converter for performing thermoelectric conversion by a temperature difference between the exhaust gas and the oxygen-containing gas.

TECHNICAL FIELD

The present invention relates to a fuel cell system including a fuelcell module for generating electrical energy by electrochemicalreactions of a fuel gas and an oxygen-containing gas, and a condenserfor condensing water vapor in an exhaust gas discharged from the fuelcell module by heat exchange between the exhaust gas and a coolant tocollect the condensed water and supplying the collected condensed waterto the fuel cell module.

BACKGROUND ART

Typically, a solid oxide fuel cell (SOFC) employs a solid electrolyte ofion-conductive oxide such as stabilized zirconia. The solid electrolyteis interposed between an anode and a cathode to form an electrolyteelectrode assembly. The electrolyte electrode assembly is interposedbetween separators (bipolar plates). In use, normally, predeterminednumbers of the electrolyte electrode assemblies and the separators arestacked together to form a fuel cell stack.

As the fuel gas supplied to the fuel cell, normally, a hydrogen gasproduced from hydrocarbon raw material by a reformer is used. Ingeneral, in the reformer, a reformed raw material gas is obtained fromhydrocarbon raw material of a fossil fuel or the like, such as methaneor LNG, and the reformed raw material gas undergoes steam reforming, forexample, to produce a reformed gas (fuel gas).

The operating temperature of the fuel cell of this type is relativelyhigh. Therefore, the exhaust gas containing the fuel gas and theoxygen-containing gas partially consumed in the power generationreaction is hot. Thus, it is desired to effectively utilize the exhaustgas. In this regard, for example, a fuel cell system disclosed inJapanese Laid-Open Patent Publication No. 2006-024430 (hereinafterreferred to as the conventional technique 1) is known.

As shown in FIG. 9, this fuel cell system includes a solid oxide fuelcell 1 a, a heat exchanger 2 a for performing heat exchange between theexhaust gas discharged from the solid oxide fuel cell 1 a and water, ahot water tank 3 a for storing water, a circulation pipe 4 a connectingthe bottom of the hot water tank 3 a and the heat exchanger 2 a andconnecting an upper portion of the hot water tank 3 a and the heatexchanger 2 a to circulate water between the hot water tank 3 a and theheat exchanger 2 a, a circulation pump 5 a provided in the circulationpipe 4 a for forcibly circulating the water, temperature detectors 6 a,7 a for detecting the temperature of the water at the inlet and theoutlet of the heat exchanger 2 a, and a control device 8 a forcontrolling the output of the circulation pump 5 a such that thetemperature of the water at the outlet of the heat exchanger 2 a becomeshigher than the temperature of the water at the inlet of the heatexchanger 2 a by a predetermined temperature or more.

Further in a fuel cell system disclosed in International Publication No.WO2007/052633 (hereinafter referred to as the conventional technique 2),as shown in FIG. 10, a solid oxide fuel cell 1 b, a heat exchanger 2 bfor performing heat exchange between the exhaust gas discharged from thesolid oxide fuel cell 1 b and water, a hot water tank 3 b for storingwater, a circulation pipe 4 b for circulating the water between the hotwater tank 3 b and the heat exchanger 2 b, a circulation pump 5 bprovided in the circulation pipe 4 b, and a control device 6 b forcontrolling the fuel utilization ratio during power generation of thesolid oxide fuel cell 1 b in correspondence with the amount of the usedhot water are provided.

Further, in a fuel cell system and a cogeneration system disclosed inJapanese Laid-Open Patent Publication No. 2003-187843 (hereinafterreferred to as the conventional technique 3), a fuel cell unit, anexhaust gas combustion unit, and a first heat exchanger unit areprovided. The fuel cell unit is connected to electric loads, andgenerates fuel cell electrical power with the fuel gas and theoxygen-containing gas, and supplies the electrical power to the electricloads. The exhaust gas combustion unit combusts the fuel gas and theoxygen-containing gas partially consumed in the fuel gas unit to producea combustion exhaust gas. The first heat exchanger unit collects heatfrom the combustion exhaust gas through a heat medium.

The fuel cell unit is operated continuously at predetermined temperatureor more so that electrical power generated in the fuel cell can besupplied to the electric loads even if no electrical power is requiredfor the electric loads. The heat is supplied to heat utilizationequipment which utilizes the heat medium.

Further, in a heat/electric power cogenerating device disclosed inJapanese Laid-Open Patent Publication No. 2002-048004 (hereinafterreferred to as the conventional technique 4), as shown in FIG. 11, aheat/electric power generator 1 c, an electrical energy storage device 2c, a heat accumulator 3 c, and a control device 4 c are provided.Electrical power generated by the heat/electric power generator 1 c issupplied to the electrical energy storage device 2 c, and stored in theelectrical energy storage device 2 c. Likewise, heat generated by theheat/electric power generator 1 c is supplied to the heat accumulator 3c, and stored in the heat accumulator 3 c. To a load 5 c which requiresenergy, electrical power is supplied from the electrical energy storagedevice 2 c, and heat is supplied from the heat accumulator 3 c.

SUMMARY OF INVENTION

However, in the conventional technique 1, if the temperature of thewater in the hot water tank 3 a becomes high, supply of the hot water isstopped or supply of the exhaust gas is stopped. Therefore, the hotexhaust gas is discharged wastefully. Further, it is difficult tocondense the water component in the exhaust gas to achieve perfectcirculation of the water required for reforming (water self-sustainingoperation).

Further, the conventional technique 2 is directed to control the fuelutilization ratio during power generation. Therefore, in the case wherethe hot water is fully stored in the hot water tank 3 b, or thetemperature of the hot water in the hot water tank 3 b becomes high,water self-sustaining operation and supply of the required electricalpower to the loads may not be performed, and the hot water may not bemaintained.

Further, the conventional technique 3 is intended to improve the energyefficiency even in the time zone when electrical power required by theloads and heat energy required by the loads are small. Therefore, in thecase where the hot water is fully stored in the hot water tank, or thetemperature of the hot water in the hot water tank is high, waterself-sustaining operation and supply of the required electrical power tothe loads may not be performed, and the hot water may not be maintained.

Further, in the conventional technique 4, the electrical energy storagedevice 2 c and the heat accumulator 3 c are provided. Therefore, theoverall size of the heat/electric power cogenerating device is large.Further, it is difficult to achieve water self-sustaining operation, andachieve the thermally self-sustaining operation where the operatingtemperature of the fuel cell is maintained only by the heat producedwithin the heat/electric power cogenerating device without supplyingheat from the outside.

The present invention has been made to solve the problems of this type,and an object of the present invention is to provide a fuel cell systemwhich makes it possible to facilitate water self-sustaining operationand thermally self-sustaining operation, and achieve improvement in thepower generation efficiency.

The present invention relates to a fuel cell system including a fuelcell module for generating electrical energy by electrochemicalreactions of a fuel gas and an oxygen-containing gas, and a condenserfor condensing water vapor in an exhaust gas discharged from the fuelcell module by heat exchange between the exhaust gas and a coolant tocollect the condensed water and supplying the collected condensed waterto the fuel cell module.

In the fuel cell system, the condenser includes an air coolingcondensing mechanism using the oxygen-containing gas as the coolant. Theair cooling condensing mechanism includes a thermoelectric converter forperforming thermoelectric conversion by a temperature difference betweenthe exhaust gas and the oxygen-containing gas.

In the present invention, the air cooling condensing mechanism includesthe thermoelectric converter for performing thermoelectric conversion bythe temperature difference between the exhaust gas and theoxygen-containing gas. Thus, the temperature difference (heat energy)between the exhaust gas and the oxygen-containing gas can be collectedas electrical energy, and improvement in the power generation efficiencyis achieved effectively.

Further, the temperature of the exhaust gas is decreased, and it becomespossible to condense the water vapor in the exhaust gas to collect thecondensed water. Thus, generation of waste heat is suppressed, and waterself-sustaining operation is facilitated. Further, since the temperatureof the oxygen-containing gas is increased, thermally self-sustainingoperation is facilitated easily.

Water self-sustaining operation herein means operation where the entireamount of water required for operating the fuel cell system is suppliedwithin the fuel cell system, without supplying additional water from theoutside. Thermally self-sustaining operation herein means operationwhere the entire amount of heat quantity required for operating the fuelcell system is supplied within the fuel cell system, and where theoperating temperature of the fuel cell system is maintained using onlyheat energy generated in the fuel cell system, without supplyingadditional heat from the outside.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a diagram schematically showing structure of a fuel cellsystem for carrying out a control method according to a first embodimentof the present invention;

FIG. 2 is a view showing a condenser of the fuel cell system;

FIG. 3 is a cross sectional view showing a thermoelectric converter ofthe condenser;

FIG. 4 is a flow chart showing a control method of the fuel cell system;

FIG. 5 is a control map of the control method;

FIG. 6 is a graph showing the relationship between the temperaturedifference among thermoelectric conversion elements and the output;

FIG. 7 is a diagram schematically showing structure of a fuel cellsystem according to a second embodiment of the present invention;

FIG. 8 is a cross sectional view showing a thermoelectric converter ofthe fuel cell system;

FIG. 9 is a diagram showing a fuel cell system according to theconventional technique 1;

FIG. 10 is a diagram showing a fuel cell system according to theconventional technique 2; and

FIG. 11 is a diagram showing a heat/electric power cogenerating deviceaccording to the conventional technique 4.

DESCRIPTION OF EMBODIMENTS

A fuel cell system 10 for carrying out a control method according to afirst embodiment of the present invention is used in variousapplications, including stationary and mobile applications. For example,the fuel cell system 10 is mounted in a vehicle.

As schematically shown in FIG. 1, the fuel cell system 10 includes afuel cell module 12 for generating electrical energy in power generationby electrochemical reactions of a fuel gas (hydrogen gas) and anoxygen-containing gas (air), a condenser 14 for condensing water vaporin an exhaust gas discharged from the fuel cell module 12 by heatexchange between the exhaust gas and a coolant to collect the condensedwater and supplying the collected condensed water to the fuel cellmodule 12, a control device 16, and a hot water tank (storage unit) 18for storing water as the coolant.

The fuel cell module 12 includes a fuel gas supply apparatus (includinga fuel gas pump) 20 for supplying a raw fuel (e.g., city gas) to thefuel cell module 12, an oxygen-containing gas supply apparatus(including an air pump) 22 for supplying an oxygen-containing gas to thefuel cell module 12, and a water supply apparatus (including a waterpump) 24 for supplying water to the fuel cell module 12.

The fuel cell module 12 includes a solid oxide fuel cell stack 32 formedby stacking a plurality of solid oxide fuel cells 30 in a verticaldirection (or a horizontal direction). The fuel cells 30 are formed bystacking electrolyte electrode assemblies 26 and separators 28. Thoughnot shown, each of the electrolyte electrode assemblies 26 includes acathode, an anode, and a solid electrolyte (solid oxide) interposedbetween the cathode and the anode. For example, the electrolyte is madeof ion-conductive solid oxide such as stabilized zirconia.

At a lower end (or an upper end) of the fuel cell stack 32 in thestacking direction, a heat exchanger 34 for heating theoxygen-containing gas before the oxygen-containing gas is supplied tothe fuel cell stack 32, an evaporator 36 for evaporating water toproduce a mixed fuel of a raw fuel (e.g., city gas) chiefly containinghydrocarbon and water vapor, and a reformer 38 for reforming the mixedfuel to produce a reformed gas are provided.

The reformer 38 is a preliminary reformer for reforming higherhydrocarbon (C₂₊) such as ethane (C₂H₆), propane (C₃H₈), and butane(C₄H₁₀) in the city gas (raw fuel) into a reformed gas chieflycontaining methane (CH₄) by steam reforming. The operating temperatureof the reformer 38 is several hundred ° C.

The operating temperature of the fuel cell 30 is high, at severalhundred ° C. In the electrolyte electrode assembly 26, methane in thefuel gas is reformed to obtain hydrogen, and the hydrogen is supplied tothe anode.

The heat exchanger 34 induces heat exchange between the partiallyconsumed reactant gas discharged from the fuel cell stack 32(hereinafter also referred to as the exhaust gas or the combustionexhaust gas) and the air as the heated fluid by allowing the exhaust gasand the air to flow in a counterflow manner for heating the air. Theexhaust gas after heat exchange is discharged into an exhaust gas pipe40, and the air after heat exchange is supplied as the oxygen-containinggas to the fuel cell stack 32.

The outlet of the evaporator 36 is connected to the inlet of thereformer 38, and the outlet of the reformer 38 is connected to a fuelgas supply passage (not shown) of the fuel cell stack 32. A main exhaustgas pipe 42 is provided for discharging the exhaust gas supplied to theevaporator 36. The main exhaust gas pipe 42 is formed integrally withthe exhaust gas pipe 40.

The condenser 14 includes an air cooling condenser (air coolingcondensing mechanism) 44 using the oxygen-containing gas as a coolant, awater cooling condenser (water cooling condensing mechanism) 46 usinghot water stored in the hot water tank 18 as a coolant. For example,each of the air cooling condenser 44 and the water cooling condenser 46has a rectangular cylinder shape, i.e., has a rectangular shape in crosssection.

A regulator valve 48 is provided in the exhaust gas pipe 40 extendingfrom the fuel cell module 12. Exhaust gas channels 50 a, 50 b arebranched at the outlet of the regulator valve 48. The air coolingcondenser 44 is connected to the exhaust gas channel 50 a, and the watercooling condenser 46 is connected to the exhaust gas channel 50 b.

The regulator valve 48 can regulate the flow rate of the exhaust gassupplied to the air cooling condenser 44 and the flow rate of theexhaust gas supplied to the water cooling condenser 46 separately. Theexhaust gas may be supplied only to the air cooling condenser 44, theexhaust gas may be supplied only to the water cooling condenser 46, orthe exhaust gas may be supplied to both of the air cooling condenser 44and the water cooling condenser 46 at predetermined flow rates.

The regulator valve 48 may change the flow rate of the exhaust gasdistributed to the exhaust gas channel 50 a, and the flow rate of theexhaust gas distributed to the exhaust gas channel 50 b to regulate theflow rates of the exhaust gas supplied to the exhaust gas channels 50 a,50 b. In the control described later, it is required to increase ordecrease the flow rates of the exhaust gas distributed to the exhaustgas channels 50 a, 50 b by increasing or decreasing the output of thefuel cell module 12 to increase or decrease the amount of the suppliedexhaust gas.

At the outlets of the air cooling condenser 44 and the water coolingcondenser 46, exhaust gas channels 52 a, 52 b for discharging theexhaust gas are provided, and condensed water channels 56 a, 56 b forsupplying the condensed water to a water container 54 are provided. Thewater container 54 stores the condensed water, and the water container54 is connected to the water supply apparatus 24 through a water channel58. After the exhaust gas channels 52 a, 52 b are merged, the exhaustgas channels 52 a, 52 b can discharge the exhaust gas to the outside ofthe condenser 14.

As shown in FIGS. 2 and 3, a thermoelectric converter 60 for performingthermoelectric conversion by a temperature difference between theexhaust gas and the oxygen-containing gas is attached to the air coolingcondenser 44. The thermoelectric converter 60 includes a plurality ofthermoelectric conversion elements 64 attached between one flat outersurface (hot temperature side) 44 a of the air cooling condenser 44 anda rectangular cylinder shaped air flow pipe (low temperature side) 62 asa passage of the air (oxygen-containing gas). As described later, thethermoelectric conversion elements 64 are capable of generating anelectromotive force by generating a temperature difference between bothends.

As shown in FIGS. 1 to 3, an air supply pipe 66 is connected to the airflow pipe 62. The oxygen-containing gas supply apparatus 22 is providedat the air supply pipe 66. The oxygen-containing gas from the air supplypipe 66 is supplied as the coolant into the air flow pipe 62, and afterthe oxygen-containing gas is heated by heat exchange with the exhaustgas, the oxygen-containing gas is supplied to the fuel cell stack 32.

A circulation pipe 68 connected to the hot water tank 18 is provided inthe water cooling condenser 46. In the circulation pipe 68, the hotwater in the hot water tank 18 is supplied as the coolant into the watercooling condenser 46 for heat exchange with the exhaust gas, and theheated hot water is returned to the hot water tank 18.

The control device 16 at least includes an electrical power comparator70 for comparing supplied electrical power (electrical energy) W with ademanded electrical power range, and a heat quantity comparator 72 forcomparing supplied heat quantity Q with a predetermined demanded heatquantity range.

The hot water tank 18 includes a water level meter (hot water leveldetector) 80 for detecting the water level of the hot water stored inthe hot water tank 18 and a thermometer (hot water temperature detector)82 for detecting the temperature of the hot water stored in the hotwater tank 18. The water container 54 includes a water level meter(condensed water level detector) 84 for detecting the water level of thecondensed water in the water container 54.

A water supply pipe 86 for receiving city water supplied from theoutside, a water discharge pipe 88 for discharging water, and a hotwater supply pipe 90 for supplying the stored hot water at apredetermined temperature are connected to the hot water tank 18. Valves92, 94, and 96 are provided in the water supply pipe 86, the waterdischarge pipe 88, and the hot water supply pipe 90, respectively.

Operation of the fuel cell system 10 will be described below.

By operation of the fuel gas supply apparatus 20, for example, a rawfuel such as the city gas (including CH₄, C₂H₆, C₃H₈, C₄H₁₀) is suppliedto the evaporator 36. Further, by operation of the water supplyapparatus 24, water is supplied to the evaporator 36, and by operationof the oxygen-containing gas supply apparatus 22, the oxygen-containinggas such as the air is supplied to the heat exchanger 34 through thethermoelectric converter 60.

In the evaporator 36, the raw fuel is mixed with the water vapor, and amixed fuel is obtained. The mixed fuel is supplied to the inlet of thereformer 38. The mixed fuel undergoes steam reforming in the reformer38. Thus, hydrocarbon of C₂₊ is removed (reformed), and a reformed gaschiefly containing methane is obtained. The reformed gas flows throughthe outlet of the reformer 38, and the reformed gas flows into the fuelcell stack 32. Thus, the methane in the fuel gas is reformed, and thehydrogen gas is obtained. The fuel gas chiefly containing the hydrogengas is supplied to the anodes (not shown).

The air supplied to the heat exchanger 34 moves along the heat exchanger34, and the air is heated to a predetermined temperature by heatexchange with the exhaust gas as described later. The air heated by theheat exchanger 34 flows into the fuel cell stack 32, and the air issupplied to the cathodes (not shown).

Thus, in each of the electrolyte electrode assemblies 26, byelectrochemical reactions of the fuel gas and the air, power generationis performed. The hot exhaust gas (several hundred ° C.) discharged tothe outer circumferential region of each of the electrolyte electrodeassemblies 26 flows through the heat exchanger 34, and heat exchangewith the air is carried out. The air is heated to a predeterminedtemperature, and the temperature of the exhaust gas is decreased. Theexhaust gas is supplied to the evaporator 36 to evaporate the water.After the exhaust gas passes through the evaporator 36, the exhaust gasis supplied from the main exhaust gas pipe 42 to the condenser 14through the exhaust gas pipe 40.

Next, a control method according to the first embodiment in thecondenser 14 will be described with reference to a flow chart shown inFIG. 4 and a control map shown in FIG. 5.

Firstly, as a demanded electrical power range for the electrical power Wsupplied from the fuel cell module 12, a demanded upper limit electricalpower Wmax and a demanded lower limit electrical power Wmin aredetermined. Further, as a demanded heat quantity range for the suppliedheat quantity Q, a demanded upper limit heat quantity Qmax and ademanded lower limit heat quantity Qmin are determined.

In FIG. 5, the “ELECTRICITY MODE” means a mode for increasing ordecreasing the amount of heat energy collected as electrical energy bythe thermoelectric converter 60. Specifically, the flow rate of theexhaust gas supplied into the air cooling condenser 44 is increased ordecreased. In contrast, “HEAT MODE” is a mode for increasing ordecreasing the amount of heat energy of the exhaust gas collected asheat energy of hot water. Specifically, the flow rate of the exhaust gassupplied into the water cooling condenser 46 is increased or decreased.

In FIG. 5, “OTHER” indicates a situation where the supplied electricalpower W exceeds the demanded electrical power range and the suppliedheat quantity Q exceeds the demanded heat quantity range. In the controlin this situation, the output is decreased, and electricity istransmitted to the power system.

In the control device 16, the supplied electrical power W is comparedwith the predetermined demanded electrical power range by the electricalpower comparator 70. If it is determined that the supplied electricalpower W is the demanded upper limit electrical power Wmax or less (YESin step S1), the routine proceeds to step S2.

In step S2, if it is determined that the supplied electrical power W isthe demanded lower limit electrical power Wmin or more (YES in step S2),the routine proceeds to step S3 for comparing the supplied heat quantityQ of the fuel cell module 12 with the demanded heat quantity range bythe heat quantity comparator 72. If it is determined that the suppliedheat quantity Q is the demanded upper limit heat quantity Qmax or less(YES in step S3), the routine proceeds to step S4.

In step S4, if it is determined that the supplied heat quantity Q is thedemanded lower limit heat quantity Qmin or more (YES in step S4), theroutine proceeds to step S5 to perform a process E. That is, thesupplied electrical power W of the fuel cell module 12 is within thedemanded electrical power range, and the supplied heat quantity Q of thefuel cell module 12 is within the demanded heat quantity range. At thistime, the flow rate of the exhaust gas supplied into the air coolingcondenser 44 is maintained, and the flow rate of the exhaust gassupplied to the water cooling condenser 46 is maintained. Thus, thesupplied electrical power W and the supplied heat quantity Q aremaintained.

Further, in step S4, if it is determined that the supplied heat quantityQ is less than the demanded lower limit heat quantity Qmin (NO in stepS4), the routine proceeds to step S6 to perform a process D. In thisprocess D, the flow rate of the exhaust gas supplied into the aircooling condenser 44 is maintained, and the flow rate of the exhaust gassupplied to the water cooling condenser 46 is increased. Thus, thesupplied electrical power W is maintained, and since the amount of theheat energy of the exhaust gas collected as the heat energy of the hotwater is increased, the supplied heat quantity Q is increased.

Further, in step S3, if it is determined that the supplied heat quantityQ exceeds the demanded upper limit heat quantity Qmax (NO in step S3),the routine proceeds to step S7 to perform a process F. In the processF, the flow rate of the exhaust gas supplied into the air coolingcondenser 44 is maintained, and the flow rate of the exhaust gassupplied to the water cooling condenser 46 is decreased. Thus, thesupplied electrical power W is maintained, and since the amount of heatenergy of the exhaust gas collected as heat energy of hot water isdecreased, the supplied heat quantity Q is decreased.

In step S2, if it is determined that the supplied electrical power W isless than the demanded lower limit electrical power Wmin (NO in stepS2), the routine proceeds to step S8 for determining whether thesupplied heat quantity Q is the demanded upper limit heat quantity Qmaxor less. If it is determined that supplied heat quantity Q is thedemanded upper limit heat quantity Qmax or less (YES in step S8), theroutine proceeds to step S9.

In step S9, if it is determined that the supplied heat quantity Q is thedemanded lower limit heat quantity Qmin or more (YES in step S9), theroutine proceeds to step S10 to perform a process B. In the process B,the flow rate of the exhaust gas supplied into the air cooling condenser44 is increased, and the flow rate of the exhaust gas supplied into thewater cooling condenser 46 is maintained.

As shown in FIGS. 2 and 3, the temperature difference between both endsof the thermoelectric conversion elements 64 attached between the outersurface 44 a of the air cooling condenser 44 and the air flow pipe 62 isincreased. As shown in FIG. 6, as the increase in the temperaturedifference between both ends of the thermoelectric conversion elements64, the outputted electrical power is increased, and the amount of heatenergy of the exhaust gas collected as electrical energy is increased asthe increase in the flow rate of the exhaust gas flowing inside the aircooling condenser 44 on the high temperature side. Thus, the suppliedelectrical power W is increased, and the supplied heat quantity Q ismaintained.

In step S9, if it is determined that the supplied heat quantity Q isless than the demanded lower limit heat quantity Qmin (NO in step S9),the routine proceeds to step S11 to perform a process A. In the processA, the flow rate of the exhaust gas supplied into the air coolingcondenser 44 is increased, and the flow rate of the exhaust gas suppliedto the water cooling condenser 46 is increased. Therefore, the suppliedelectrical power W and the supplied heat quantity Q are increased.

Further, in step S8, if it is determined that the supplied heat quantityQ exceeds the demanded upper limit heat quantity Qmax (NO in step S8),the routine proceeds to step S12 to perform a process C. In the processC, the flow rate of the exhaust gas supplied into the air coolingcondenser 44 is increased, and the flow rate of the exhaust gas suppliedto the water cooling condenser 46 is decreased. Thus, the suppliedelectrical power W is increased, and the supplied heat quantity Q isdecreased.

Further, in step S1, if it is determined that the supplied electricalpower W exceeds the demanded upper limit electrical power Wmax (NO instep S1), the routine proceeds to step S13. In step S13, if it isdetermined that the supplied heat quantity Q is the demanded upper limitheat quantity Qmax or less (YES in step S13), the routine proceeds tostep S14. In step S14, if it is determined that the supplied heatquantity Q is the demanded lower limit heat quantity Qmin or more (YESin step S14), the routine proceeds to step S15 to perform a process H.

In the process H, the supplied electrical power W of the fuel cellmodule 12 exceeds the demanded upper limit electrical power Wmax, andthe supplied heat quantity Q of the fuel cell module 12 is within thedemanded heat quantity range. At this time, the flow rate of the exhaustgas supplied into the air cooling condenser 44 is decreased, and theflow rate of the exhaust gas supplied to the water cooling condenser 46is maintained. Thus, the supplied electrical power W is decreased, andthe supplied heat quantity Q is maintained.

In step S14, if it is determined that the supplied heat quantity Q isless than the demanded lower limit heat quantity Qmin (NO in step S14),the routine proceeds to step S16 to perform a process G. In the processG, the flow rate of the exhaust gas supplied into the air coolingcondenser 44 is decreased, and the flow rate of the exhaust gas suppliedto the water cooling condenser 46 is increased. Thus, the suppliedelectrical power W is decreased, and the supplied heat quantity Q isincreased.

In step S13, if it is determined that the supplied heat quantity Qexceeds the demanded upper limit heat quantity Qmax (NO in step S13),the routine proceeds to step S17 to perform a process I. That is, thesupplied electrical power W of the fuel cell module 12 exceeds thedemanded upper limit electrical power Wmax, and the supplied heatquantity Q of the fuel cell module 12 exceeds the demanded upper limitheat quantity Qmax.

Thus, in the control of the process I, the output is decreased, andelectrical energy is transmitted to the power system. The output isdecreased by decreasing the flow rate of the fuel gas and the flow rateof the oxygen-containing gas supplied to the fuel cell module 12. Thesupplied electrical power W can be consumed by supplying electricalpower to locations requiring electrical power through the power system.

In the first embodiment, the condenser 14 includes the air coolingcondenser 44 using the oxygen-containing gas as the coolant. The aircooling condenser 44 includes the thermoelectric converter 60 forperforming thermoelectric conversion by the temperature differencebetween the exhaust gas and the oxygen-containing gas. Thethermoelectric converter 60 includes the thermoelectric conversionelements 64 attached between the air cooling condenser 44 on the hightemperature side and the air flow pipe 62 on the low temperature side.

Thus, as shown in FIGS. 2 and 3, the temperature difference (heatenergy) between the exhaust gas flowing through the air coolingcondenser 44 and the oxygen-containing gas flowing through the air flowpipe 62 can be collected as electrical energy, and improvement in thepower generation efficiency is achieved effectively.

Further, the temperature of the exhaust gas is decreased, and it becomespossible to condense the water vapor in the exhaust gas to collect thecondensed water. Thus, generation of waste heat is suppressed, and waterself-sustaining operation is facilitated. Further, since the temperatureof the oxygen-containing gas is increased, thermally self-sustainingoperation is facilitated easily.

Water self-sustaining operation herein means operation where the entireamount of water required for operating the fuel cell system 10 issupplied within the fuel cell system 10, without supplying additionalwater from the outside. Thermally self-sustaining operation herein meansoperation where the entire amount of heat quantity required foroperating the fuel cell system 10 is supplied within the fuel cellsystem 10, and where the operating temperature of the fuel cell system10 is maintained using only heat energy generated in the fuel cellsystem 10, without supplying additional heat from the outside.

Further, the fuel cell system 10 includes the hot water tank 18 forstoring hot water, and the condenser 14 includes the water coolingcondenser 46 using the hot water stored in the hot water tank 18 as thecoolant. In the structure, the heat energy of the exhaust gas can becollected in the hot water tank 18, and improvement in the energyefficiency is achieved. Further, the temperature of the exhaust gas isdecreased, and it becomes possible to condense the water vapor in theexhaust gas to collect the condensed water. Thus, generation of wasteheat is suppressed, and water self-sustaining operation is facilitated.

In this manner, since the air cooling condenser 44 and the water coolingcondenser 46 are selectively used, or used in combination, the heatenergy of the exhaust gas can be collected efficiently. Thus, waterself-sustaining operation is achieved, and the temperature of theexhaust gas is decreased. Further, the demanded electrical power can besupplied reliably without being affected by the power generation outputregardless of the state of the hot water, i.e., the amount of the storedhot water.

Further, since operation of the fuel cell module 12 does not rely on thecapacity of the hot water tank 18, the fuel cell module 12 can bestarted and stopped less frequently, or as few times as possible. Thus,improvement in the power generation efficiency is achieved easily.

Further, the control device 16 at least includes the electrical powercomparator 70 for comparing supplied electrical power W and thepredetermined demanded electrical power range, and the heat quantitycomparator 72 for comparing the supplied heat quantity Q with thepredetermined demanded heat quantity range.

Therefore, by the air cooling condenser 44 and the water coolingcondenser 46, the heat energy can be collected from the exhaust gas.Thus, water self-sustaining operation is achieved, and the temperatureof the exhaust gas is decreased. Further, the demanded electrical powercan be supplied reliably without being affected by the power generationoutput regardless of the state of the hot water, i.e., the amount of thestored hot water.

Further, the hot water tank 18 includes the water level meter 80 fordetecting the water level of the stored hot water, and the thermometer82 for detecting the temperature of the stored hot water. Thus, thestate of the stored hot water, i.e., the amount (water level) of thestored hot water and the temperature of the stored hot water can bedetected reliably.

Further, the condenser 14 includes the water container 54 for storingthe condensed water and the water level meter 84 for detecting the waterlevel of the condensed water in the water container 54. Thus, the stateof the condensed water, i.e., the amount of the condensed water can bedetected reliably.

Further, the fuel cell system 10 includes the regulator valve 48 forregulating the flow rate of the exhaust gas supplied to the air coolingcondenser 44 and the flow rate of the exhaust gas supplied to the watercooling condenser 46. Thus, the amount of the exhaust gas supplied tothe air cooling condenser 44 and the amount of the exhaust gas suppliedto the water cooling condenser 46 can be regulated suitably.Accordingly, water self-sustaining operation is facilitated, and theelectrical power required for the load can be supplied. Further, thetemperature of the stored hot water can be maintained.

Further, the fuel cell module 12 includes the fuel cell stack 32, theheat exchanger 34, the evaporator 36, and the reformer 38. Thus, inparticular, the present invention is optimally applicable to the fuelcell module 12 performing steam reforming.

Further, the fuel cell module 12 is a solid oxide fuel cell (SOFC)module. Thus, the fuel cell module 12 is optimally used in fuel cellsystems operated at high temperature.

FIG. 7 is a diagram schematically showing a fuel cell system 100according to a second embodiment of the present invention.

The constituent elements that are identical to those of the fuel cellsystem 10 according to the first embodiment are labeled with the samereference numeral, and description thereof will be omitted.

As shown in FIG. 8, the fuel cell system 100 includes one or more, e.g.,two air cooling fans 102 attached on the outer surface 44 a of the aircooling condenser 44. A plurality of thermoelectric conversion elements64 of a thermoelectric converter 60 are interposed between the outersurface 44 a of the air cooling condenser 44 and the air cooling fans102.

Thus, in the air cooling condenser 44, the external air(oxygen-containing gas) forcibly supplied from the air cooling fans 102cools the exhaust gas discharged from the fuel cell stack 32, and thetemperature difference among the thermoelectric conversion elements 64can be generated.

In this manner, in the second embodiment, the same advantages as in thecase of the first embodiment are obtained. For example, heat energy ofthe exhaust gas flowing through the air cooling condenser 44 can becollected suitably as electrical energy, and improvement in the powergeneration efficiency is achieved.

Although certain embodiments of the present invention have been shownand described in detail, it should be understood that various changesand modifications may be made to the embodiments without departing fromthe scope of the invention as set forth in the appended claims.

The invention claimed is:
 1. A fuel cell system comprising: a fuel cellmodule for generating electrical energy by electrochemical reactions ofa fuel gas and an oxygen-containing gas; and a condenser for condensingwater vapor in an exhaust gas discharged from the fuel cell module byheat exchange between the exhaust gas and a coolant to collect thecondensed water and supplying the collected condensed water to the fuelcell module; a hot water storage unit for storing hot water; a controldevice; and a regulator valve, wherein the condenser includes an aircooling condensing mechanism using the oxygen-containing gas as thecoolant; and a water cooling condensing mechanism using the hot waterstored in the hot water storage unit as the coolant, the air coolingcondensing mechanism includes a thermoelectric converter for performingthermoelectric conversion by a temperature difference between theexhaust gas and the oxygen-containing gas, the hot water storage unitincludes a hot water level detector for detecting a water level of thehot water; and a hot water temperature detector for detecting atemperature of the hot water, the regulator valve is configured toseparately regulate a flow rate of the exhaust gas supplied to the aircooling condensing mechanism and a flow rate of the exhaust gas suppliedto the water cooling condensing mechanism, the control device includesan electrical power comparator for at least comparing suppliedelectrical power with a predetermined demanded electrical power range;and a heat quantity comparator for comparing supplied heat quantity witha predetermined demanded heat quantity range, wherein the control devicecontrols the regulator valve based on a result from the electrical powercomparator and a result from the heat quantity comparator obtained afterthe result from the electrical power comparator.
 2. The fuel cell systemaccording to claim 1, wherein the condenser includes a water containerfor storing the condensed water and a condensed water level detector fordetecting a water level of the condensed water in the water container.3. The fuel cell system according to claim 1, further comprising a fuelcell stack formed by stacking a plurality of fuel cells, the fuel cellseach including an electrolyte electrode assembly and a separator stackedtogether, the electrolyte electrode assembly including an anode, acathode, and an electrolyte interposed between the anode and thecathode; a heat exchanger for heating an oxygen-containing gas beforethe oxygen-containing gas is supplied to the fuel cell stack; anevaporator for evaporating water to produce a mixed fuel of water vaporand a raw fuel chiefly containing hydrocarbon; and a reformer forreforming the mixed fuel to produce the fuel gas.
 4. The fuel cellsystem according to claim 1, wherein the fuel cell module is a solidoxide fuel cell module.